The present invention relates to a process and apparatus for the treatment of water and to a device for the distribution of water to be treated into and for the recovery of washing liquid from a water treatment apparatus. The present invention is particularly applicable to water treatment apparatuses of the type including at least one layer of granular filtering material in which at least one of the filtering materials is of low volume mass, for example treated carbons, such as treated anthracite activated charcoal, pumice stone, expanded clay, etc. The present invention is particularly directed to the treatment of all types of water, for example sewage, industrial waste, etc., and the present invention is not intended to be limited to the treatment of any particular type or quality of water.
Filter apparatuses employing filter material of low volume mass are known and generally are in the form of a parallelepipedal vat or tank having a bottom equipped with drainage channels and intended to support the filter material or materials. The water to be filtered is distributed by any means over the mass of filter material. As is known, it is periodically necessary to wash such filter material to remove therefrom clogging material retained during the water treatment operation. The most effective washing technique for this type of filter material involves a backwashing technique consisting of an initial unclogging operation by the simultaneous injection of washing liquid, such as water, and gas, such as air, in a countercurrent or upward direction through the bottom support and through the mass of filtering material, and then a rinsing operation by injecting water only as a rinsing liquid upwardly through the filtering material to remove therefrom impurities and clogging material loosened from the filtering material by the previous operation. The washing liquid and loosened clogging materials are then evacuated into a channel normally arranged along the filter apparatus and connected to a drain.
This washing operation, applied to a filter apparatus having a filtering medium of a material which is light in weight, for example treated anthracite, presents certain inherent operational difficulties. Thus, for example with treated anthracite having a volume mass on the order of 1500 kg/m.sup.3, while that of sand is 2500 kg/m.sup.3, during the unclogging operation of passing washing liquid and gas upwardly through the filter material, there occurs the phenomenon that a substantial portion of the filtering material is entrained and lifted or floated upwardly within the tank. Thus, when the level of the washing liquid-gas emulsion reaches the overflow of the channel for recovery of the wash water, a portion of the filter material is also discharged or evacuated to the drain along with the wash water. This of course is an undesirable occurrence.
To avoid this disadvantage, two solutions have previously been proposed. The first such proposal involves positioning the overflow of the wash water recovery channel at a height of 1.5 to 2.0 meters above the upper level of the mass of filter material. Then, prior to the simultaneous injection of washing liquid and gas, the filter apparatus is emptied of liquid until the level of water in the device essentially matches that of the mass of filtering material. Thereafter, the unclogging operation by the simultaneous injection of washing liquid and gas is performed. During this operation, the water level in the filter rises to the level of the overflow of the wash water recovery channel. Thus, the wash water is stored during the unclogging operation without any discharge of such water into the recovery channel. A gas layer which has been formed beneath the bottom support is drained off, and it is necessary to wait until the filter material entrained in the washing liquid-air emulsion is separated, i.e. by settling. Then the filter material is rinsed with washing liquid alone, and such washing liquid overflows into the channel placed at the upper part of the filter apparatus. This solution has, however, certain drawbacks. Thus, the operation of rinsing with washing liquid alone, intended to evacuate into the recovery channel any impurities or clogging material detached from the filter material, requires a major flow of water because of the high position of the recovery channel. Additionally, the complete evacuation of the impurities or clogging material from the interior of the tank takes a very long time.
The second known solution makes it possible to avoid the above-disadvantage during the operation of rinsing with washing liquid alone, and such solution involves preceding as discussed above during the unclogging operation with injection of washing liquid and gas, but of evacuating the rinse water during the rinsing operation by means of a valve which is closed during the unclogging operation. However, this arrangement presents certain other disadvantages. Thus, applied to an industrial operational filter apparatus of classic dimensions, for example 10 meters long and 4 meters wide, this solution requires a 4 meter wide motorized control valve, and is therefore very expensive. In addition, for a filter of this size, the length of the rinsing operation is considerably increased, the maximum course of the rinse water in this arrangement being 10 meters.